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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 4, AUGUST 2003
A CMOS-MEMS Mirror With
Curled-Hinge Comb Drives
Huikai Xie, Member, IEEE, Yingtian Pan, and Gary K. Fedder, Senior Member, IEEE
Abstract—A micromirror achieves up to 4 7 angular displacement with 18 Vdc by a comb-drive design that uses vertical
angled offset of the comb fingers. Structures are made from a combination of CMOS interconnect layers and a thick underlying silicon layer. Electrical isolation of the silicon fingers is realized with a
slight silicon undercut etch, which disconnects sufficiently narrow
pieces of silicon under the CMOS microstructures. The 1 mm by 1
mm micromirror is made of an approximately 40 m-thick singlecrystal silicon plate coated with aluminum from the CMOS interconnect stack. The mirror has a peak-to-peak curling of 0.5 m.
Fabrication starts with a conventional CMOS process followed by
dry-etch micromachining steps. There is no need for wafer bonding
and accurate front-to-backside alignment. Such capability has potential applications in biomedical imaging, optical switches, optical
scanners, interferometric systems, and vibratory gyroscopes.
[975]
Index Terms—Curled-hinge comb drive, high-aspect-ratio, micromirror, out-of-plane actuation.
I. INTRODUCTION
M
ICROMIRRORS have obtained extensive attention recently because of their applications in optical switches
and displays. They are also used in a wide range of other applications, such as interferometric systems [1], optical spectroscopy
[2], aberration correction [3] and biomedical imaging [4], [18].
The requirements of micromirrors vary with different applications. Flatness, roughness and reflectivity are the common metrics across most applications. For optical displays, fill factor
and pixel size are the most important parameters. For optical
switches, speed, maximum tilt angle and power consumption
are primary requirements.
Micromirrors have been demonstrated by using different
micromachining processes. A successful example of surface-micromachined micromirrors is Texas Instruments’ digital
mirror device (DMD) [5]. Scanning polysilicon micromirrors
have also been used in barcode readers [6]. In order to overcome
the small out-of-plane actuation range limited by the substrate
Manuscript received December 13, 2002; revised April 10, 2003. This
work was supported in part by DARPA and the U.S. AFRL, under agreement F30602-99-2-0545, NIH Contract NIH-1-R01-DK059265-01, and the
Whitaker Foundation Contract 00-0149. Subject Editor D. Cho.
H. Xie was with the Department of Electrical and Computer Engineering,
Carnegie Mellon University, Pittsburgh, PA 15213 USA. He is now at the
Department of Electrical and Computer Engineering, University of Florida,
Gainesville, FL 32611 USA (e-mail: hkxie@ece.ufl.edu).
Y. Pan is with the Departments of Bioengineering, State University of New
York at Stony Brook, Stony Brook, NY 11794 USA.
G. K. Fedder is with the Dept of Electrical & Computer Engineering and
The Robotics Institute, Carnegie Mellon University, Pittsburgh, PA 15213 USA
(e-mail: fedder@ece.cmu.edu).
Digital Object Identifier 10.1109/JMEMS.2003.815839
electrode, vertical comb drives have been explored using various techniques to create uneven stationary and movable comb
fingers. Selvakumar et al. employed a bulk/poly-silicon trench
refill technology [7]. Yeh et al. reported a design with movable
polysilicon comb fingers staggered above stationary bulk-Si
comb fingers [8]. Syms demonstrated a 3-D microscanner
using a surface-tension self-assembling technique [9]. All the
mirrors mentioned above are made of thin-film materials. It
is well known that it is difficult to achieve large flat thin-film
mirrors with large tilt range due to the residual stress existing
in thin-film structures. Bulk-micromachining processes have
been used to make sidewall mirrors [10], but the sidewall
angle, smoothness and area efficiency are concerns. Another
choice is to combine surface- and bulk-micromachining
processes. A single-crystal silicon (SCS) based micromirror
was fabricated by using silicon-on-insulator (SOI) wafers and
two-side alignment [11]. The stator and rotator comb fingers
of the micromirror are at different height levels. An optical
scanning angle of 24.9 was achieved with a 171 Vrms voltage
at the resonant frequency (34 kHz) of the micromirror. Similar
to the flip-chip thin-film micromirror array reported in [12],
-by-460
SCS mirror was assembled on top of
a 460
an electrostatic polysilicon actuator to achieve a flat surface
above the substrate,
[13]. The polysilicon actuator was 60
raised by self-assembled Micro-Elevator by Self-Assembly
(MESA) structures [14]. The assembled SCS micromirror can
perform two-dimensional (2-D) scanning with a maximum
. The same group reported a
optical scanning angle of
SCS micromirror with integrated angular comb drive actuation
[15]. The tilt of the comb drives was realized by a hard-baked
photoresist hinge, and the device was built on SOI wafers.
Kwon et al. also demonstrated an SOI vertical comb actuator
for moving microlenses [16].
The authors demonstrated a 1 mm by 1 mm, flat SCS micromirror that rotates 17 with thermal actuation at 12 mA current [17] and assembled the micromirror in an endocopic optical
coherence tomography imaging system for the transverse laser
scanning [18]. However, the large tilt angle at the rest position
makes it difficult to align and perform symmetric scanning, and
the scanning speed is limited by the thermal actuation.
The micromirror described in this paper is made of
single-crystal silicon (SCS) and coated with aluminum [19].
A deep reactive-ion-etch (DRIE) CMOS-MEMS process
[20], which is based on a previous thin-film CMOS-MEMS
process [21], is employed. The micromachining process steps
are performed after the foundry CMOS process steps are
completed, and are completely compatible with conventional
CMOS processes. Unlike the lateral comb drives, the stator
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XIE et al.: A CMOS-MEMS MIRROR WITH CURLED-HINGE COMB DRIVES
451
Fig. 2. Schematic of a curled-hinge comb drive.
of silicon [see Fig. 1(c)]. At the end of this step, a thick SCS
layer remains underneath the CMOS layer, resulting in a flat released microstructure. Finally, a brief isotropic silicon etch is
performed [see Fig. 1(d)]. Any beam with a half-width less than
the silicon undercut will have no SCS layer underneath. This
type of beam can be used to form electrically isolated SCS islands, purposefully curled-up structures or z-compliant springs.
Fig. 1. DRIE CMOS-MEMS process flow: (a) backside etch; (b) oxide etch;
(c) deep Si etch; and (d) Si undercut.
and rotor comb fingers of the z-axis comb drive do not lie
in the same plane, and therefore large electrically actuated
out-of-plane displacement is achieved. Though similar to the
staggered comb drive reported in [11], this new design realizes
the uneven stator and rotor comb fingers by using the curling of
multilayer thin-film beams. Therefore, high-accuracy two-side
alignment and expensive SOI wafers are not necessary.
II. DRIE CMOS-MEMS PROCESS
It is well known that thin-film deposition processes generate
residual stress and stress gradients, which cause curling. This
curling limits the useful size of micromirrors. The small gap
generated by the sacrificial layer present in surface-micromachined mirrors restricts their tilt range. In order to overcome
the nonplanarity of thin-film CMOS microstructures, a thick
bulk-silicon mirror is desirable. The present mirror introduces
a single-crystal silicon (SCS) layer underneath CMOS multilayer structures in such a way that the mechanical properties are
dominated by the SCS layer and electrical connections are provided by the CMOS interconnect metal layers. A backside etch
is used to eliminate the remaining silicon substrate under the
microstructures. Therefore, the microstructures can move vertically for hundreds of microns before encountering a limit stop.
The process flow is given in Fig. 1, and starts with the Agilent
CMOS process. However, any CMOS process
3-metal 0.5
can be used. The backside silicon DRIE step leaves a 10
to 100
-thick SCS membrane [see Fig. 1(a)]. This step controls the thickness of the microstructure and forms a cavity (
deep) that allows the microstructure to move freely in a
wide range. The depth of the cavity is determined by the thickness of the CMOS chips. Next, an anisotropic dielectric etch is
performed from the frontside [see Fig. 1(b)], followed by DRIE
III. CURLED-HINGE COMB DRIVE
The metal-1 beam shown in Fig. 1(d) has only thin layers of
interconnect aluminum and dielectrics. This beam curls up after
it is released because the residual stress of the top aluminum is
tensile and the residual stress of the underlying oxide is compressive. Judiciously placing and sizing the metal-1 beams that
connect to the comb fingers creates a “curled-hinge” comb drive
with the stationary and movable fingers at different levels. This
concept of the curled-hinge comb drive is illustrated in Fig. 2.
The comb drive has two parts: a set of tilted comb fingers and
a set of level comb fingers. The tilted combs are composed of
a curled metal-1 mesh as a hinge and an array of tilted fingers
with a thick underlying SCS layer. The silicon substrate underneath the metal-1 mesh is completely undercut during the deep
Si etch because the metal-1 mesh consists of only narrow beams.
Therefore, the SCS chunks under the tilted comb fingers are
electrically isolated from the silicon substrate. The SCS chunks
then can be wired through CMOS contacts to any place on the
chip, e.g., a bonding pad. When a voltage is applied to the comb
drive, the tilted and level comb fingers will tend to align and thus
an electrostatic rotational torque is generated. The initial angle
of the tilted comb fingers depends on the length of the metal-1
mesh and can be 45 or even larger, so torque through a large
rotation angle can be designed.
As shown in Fig. 2, the curling of the thin-film hinge lifts
up the fingers on one side of the comb drive, allowing large
out-of-plane actuation range. The out-of-plane angle of the tilted
fingers depends on the hinge length. It was measured that the
for
radius of curvature of metal-1 beams was about 280
3-metal CMOS process by using a standard
Agilent 0.5
post-CMOS micromachining process [21]. Note that the plasma
thick aluetch of oxide [see Fig. 1(b)] also removes 0.1–0.2
minum through ion milling. Metal-1 beams with different aluminum thicknesses will have different curvature. The thinner
the aluminum layer, the smaller the radius of curvature of the
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 4, AUGUST 2003
metal-1 beam. It is possible to intentionally increase the ion
milling of the aluminum layer to obtain large curvature. Care
must be taken not to damage vias.
As shown in Fig. 2, the fingers tilt along the tangential di,
rection at the tip of the curled hinge. Therefore,
is the rawhere is the tilt angle, is the hinge length and
.
dius of curvature. Assume a 25 tilt angle and
. The stiffness of the hinge also depends on
Thus,
the hinge width and thickness. The hinge is made of metal-1
thick. The pitch of the curled-hinge
beams which are 1.9
. By considering the holes on the hinge
fingers is about 15
for silicon undercut, the effective single hinge width is about
. The effective Young’s Modulus of metal-1 beams is 69
7.5
GPa [26]. Therefore, the stiffness of a single hinge is given by
, where is the Young’s modare the width, thickness and length of a single
ulus, , and
hinge, respectively. Since the thickness of the SCS layer is typand the finger length is approximately 150
,
ically 40
this portion of the fingers can be considered as rigid. Thus, the
equivalent torsional stiffness with respect to the y-axis can be
approximated as
(1)
Fig. 3. Topology of the micromirror design. The combs labeled X1, X1 , X2
and X2 have their curled-hinge comb fingers anchored, and therefore pull up
on the mirror. The combs labeled Y1, Y1 , Y2 and Y2 have their level comb
fingers anchored, and therefore pull down on the mirror.
is the length of the SCS part of the finger. If only 5
where
tilt angle is needed, then
. Accordingly,
and
are 243 N/m and
, respectively.
for
5 is about 30 times greater than that for 25 .
IV. MIRROR DESIGN
The curled-hinge comb drive in Fig. 2 shows the level comb
fingers anchored on the substrate. Alternatively, the tilted comb
fingers can be anchored. The top view of the micromirror design is illustrated schematically in Fig. 3(a), taking advantage of
these two anchor options. The comb drives with anchors on the
curled side are denoted as “ ” and those anchored on the level
comb drives pull the mirror
side are denoted as “ ”. The
comb drives pull the mirror down. The comb
up, and the
drives are distributed on two sides of the mirror to obtain bidirectional balanced rotation. The two sets of each comb drive
double the y-axis torque and zero the net z-axis force. The comb
rotate the mirror clockwise, and
rotate
drives
the mirror counterclockwise, as shown in Fig. 3(b) and (c).
V. TORSIONAL SPRING DESIGN
A torsional beam is simple and robust as a suspension, but
the utilization of chip area is poor. If a folded beam flexure is
used, the z-axis compliance as well as the out-of-plane compliance is obtained. It has been found that a curled, folded beam
flexure increases the resonant frequency of the z-axis mode,
, by a factor of 3 [22]. Thermomechanical FEM simulation
[23] also shows that the microstructure reported in [22] lowers
from 1.3 for flat spring beams to 0.4 for curled spring
is the resonance frequency of the torsional
beams, where
mode. Therefore, a curled, folded spring design is employed to
realize torsional compliance, as shown in Fig. 4. The vertical
Fig. 4. Schematic of a folded torsional spring.
curling of the spring beams is generated by using the same technique as that for the curled-hinge comb drive discussed above.
Regarding the analytical analysis of spring constants of folded
mechanical flexures, interested readers may refer to [25].
One of the design concerns is that the stiffness of the comb
drive hinge should be much greater than that of the torsional
spring of the mirror plate. The mirror plate can be considered as
a thin sheet since its lateral dimensions are much larger than its
thickness. Therefore, the moment of inertia of the mirror plate
along the y-axis is given by
(2)
,
and
are respectively the mass, mass
where , ,
density, width, length and thickness of the mirror plate. Since
the majority of the mirror plate is silicon, can be approx.
imated as the mass density of silicon which is 2700
XIE et al.: A CMOS-MEMS MIRROR WITH CURLED-HINGE COMB DRIVES
453
O
A
Fig. 5. Cross section of the curled-hinge comb drive for rotation analysis. is the rotation center, is the intersection of the extended lines of the tilted finger
top and the mirror surface, is the intersection of the tilted finger top with the extended line of the level finger edge and C is the upper-right corner of the level
finger.
B
Using (2), for a mirror plate 1 mm by 1 mm by 40
thick,
. Thus, if a torsional resonant
frequency of 200 Hz is desired, the required spring constant
of the torsional spring should be equal to
, which is less than one fifth of
the torsional spring constant of a single hinge of the comb drives
with 25 initial tilt angle [see (1)]. Typically, each group of the
comb drives has about 20 curled-hinge fingers. So, the hinges of
the comb drives are about two orders of magnitude stiffer than
the torsional spring, implying that the torsional spring could be
one order of magnitude stiffer, or the resonant frequency of the
mirror could be improved to 650 Hz.
If even higher speed is needed, reducing the initial angle from
25 to 5 will increase the hinge stiffness by a factor of 30,
as discussed in the previous section. The number of curledhinge fingers can also be increased by a factor of 2 to 5. Therefore, 5 kHz to 10 kHz resonant frequency is achievable. Further
increasing speed is still possible, but at the price of reducing
mirror size or increasing chip size.
VI. ELECTROSTATIC FORCE ANALYSIS
Fig. 5 shows the cross section of a curled-hinge comb drive.
The finger of the comb drive with curled metal-1 hinge tilts
out-of-plane with an angle, , at the rest position. According to
the rough analysis in the previous section, it is reasonable to assume that the assembly of the tilted finger, the curved thin-film
connection and the mirror is rigid with respect to the torsional
spring flexure. Thus, the whole assembly rotates at the same
angle, .
In order to estimate the performance of the curled comb drive,
the electrostatic force must be first calculated. The electrostatic
force in the z-direction is given by
torque associated with the force and the torsional stiffness,
of the folded flexure determine the rotation angle
,
(4)
, is the resonance frequency and
, is the moment of inertia of the mirror plate.
For sizing purposes during design, a parallel-plate approximation is used to estimate capacitance
where
(5)
is the vacuum permittivity
,
where
is the comb finger gap, and
is the overlap area between
, sections
the level and tilted fingers. When calculating
of the right edge of the level finger and its extension line are
, where
used, as shown in Fig. 5. Note that
. There are three cases
when calculating the overlap area.
1) When is small, the tilted finger completely covers the
upper-right corner of the level finger but does not exceed
the lower-right corner of the level finger. In this case, the
overlap area is given by
(6)
is the overlap at the
where
right edge of the level finger, is the overlap at the rest
position, and plus is the z-axis displacement at the
edge. , and are given by
(7)
(8)
(3)
is the
where is the sidewall capacitance per comb finger,
number of comb fingers, is the applied voltage and is the
distance from the force source to the rotation center, O. The
(9)
is the
where is the thickness of the comb fingers,
is the distance
distance from point O to point A, and
from point A to point B.
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TABLE I
DESIGN PARAMETER VALUES OF THE CURLED-HINGE COMB FINGER (SEE FIG. 5)
Fig. 7. Rotation angle dependence on geometric parameters. (a) Thickness
of the comb fingers. (b) Distance from the comb drives to the central
rotational axis.
3) When becomes even larger, the upper-right corner of
, where
the level finger will be exposed, i.e.,
is the angle when
.
is obtained by numerically
solving the following equation:
(11)
Fig. 6. Rotation analysis of the curled-hinge comb drive. (a) Capacitance
versue angle. (b) Capacitance gradient with respect to angle. (c) Rotation angle
versus applied voltage.
2) When
becomes larger, the tilted finger exceeds the
, but still covers the
lower-right corner, i.e.,
upper-right corner of the level finger. The total overlap
area is subtracted by the exceeded area and is given by
(10)
In this case, the total overlap area must also exclude the exposed area and is given by
(12)
The parameter values for the designed mirror are listed
and
are obin Table I. Evaluating (5)to (12),
tained as plotted in Fig. 6(a) and (b). The capacitance reaches
its maximum value at a rotation angle of 5 . Substituting
into (4) and solving the torque balance equation, i.e.,
XIE et al.: A CMOS-MEMS MIRROR WITH CURLED-HINGE COMB DRIVES
455
Fig. 8. SEMs of the micromirror with curled-hinge comb drives. (a) Top view of a released electrostatic micromirror. (b) Curled-hinge comb drive. (c) Close-up
of tilted comb fingers.
, gives the static relationship between
the rotation angle and applied voltage, as shown in Fig. 6(c). It
is clear that the rotation of the mirror changes rapidly around 5
V and then saturates at about 4.8 .
Rotation angle dependencies on the finger thickness and the
distance of the comb drive to the rotational axis are plotted in
Fig. 7. As shown in Fig. 7(a), the rotation angle is very sensitive
to finger thickness. The maximum rotation angle is doubled as
to 90
. Placing
the finger thickness increases from 50
comb drives closer to the rotational axis also significantly increases the maximum rotation angle without increasing drive
voltage, as shown in Fig. 7(b).
VII. EXPERIMENTAL RESULTS
The top view of a released 1 mm by 1 mm micromirror is
shown in Fig. 8(a). A view of a curled-hinge comb is shown in
Fig. 8(b). The lengths of the metal-1 mesh and the SCS fingers
are 120
and 150
, respectively. The thickness of the SCS
. Fig. 8(c) is a close-up of the comb finchunks is about 40
is used to assure the
gers. An initial undercut of about 0.2
complete undercut of silicon underneath the metal-1 meshes and
z-compliant springs. To further guarantee the electrical isolation of the silicon chunks, an n-doped well (with p-substrate) is
placed underneath the metal-1 meshes and z-compliant springs.
All of the tested devices had electrically isolated combs.
The profile of the mirror surface was characterized by using a
Wyko NT2000 3D optical profiler. The measured peak-to-peak
(see Fig. 9). Simply
curling across the entire mirror is 0.5
increasing the thickness of the SCS layer will reduce curling.
Alternatively, stress in custom thin-film layers may be tuned to
increase flatness. For instance, it was found that metal-1 beams
with field oxide removed and just gate oxide left are much flatter
than those with field oxide underneath [26].
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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 4, AUGUST 2003
Fig. 9. Contour plot of the mirror profile measured with an optical profiler.
Fig. 11.
Frequency response measured by using an optical microvision system.
VIII. CONCLUSION
A large, flat micromirror with large out-of-plane actuation
has been experimentally demonstrated. The 9.4 rotation angle
of the 1 mm by 1 mm mirror is limited by the implemented comb
design. The mirror topology can be optimized for larger rotation
angle or larger size. For example, by simply moving the curledhinge comb drives from the two sides of the mirror closer to
the central axis of the mirror, much larger rotation angle will be
achieved. The mirror has adequate tilt angle and bandwidth for
potential application in certain optical switches, optical scanners
and medical imaging.
Fig. 10.
The mirror rotation angle versus applied voltage.
Fig. 10 shows the measured rotation angle versus the applied voltage. Applying 18 V results in a tilt of 4.7 . Since the
width of the entire device is 1.5 mm, the z-displacement at the
. The center plate can rotate to both
comb-finger tips is 62
sides and therefore a total rotation angle of 9.4 can be achieved.
The rotation angle saturates above 20 V. The measured saturation angle is in good agreement with the analysis result plotted in
Fig. 6(c). However, the measured voltage corresponding to the
rapid rotation angle change is 15 V, instead of 5 V in the analysis. This is because the analysis assumed straight sidewalls for
all the comb fingers. In reality, the cross section of the comb
fingers consists of a wider metal/oxide layer on the top of a narrower but much thicker SCS layer. The fringing effects and the
slope of the SCS sidewalls will change the capacitance gradient,
which is not included in the simplified model shown in Fig. 5.
The frequency response measured by using an optical microvision system [24] is plotted in Fig. 11. The torsional resonant
frequency is 233 Hz at very low applied drive voltage. The resonance frequency of this type of micromirrors can be increased
to 10 kHz range, if needed, by increasing the spring stiffness
and drive voltage. Of course, the stiffness of the hinges should
be also increased accordingly, and the mirror size may have to
be reduced, as discussed in a previous section. For high-resonance micromirrors, operating at the resonance frequency may
be required to achieve desired scanning range.
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Huikai Xie (M’00) received the M.S. degree in
electrooptics from Tufts University, Medford,
MA, in 1998 and the Ph.D. degree in electrical
and computer engineering from Carnegie Mellon
University, Pittsburgh, PA, in 2002. He also received
the B.S. and M.S. degrees in electronic engineering
from Beijing Institute of Technology, China.
From 1992 to 1996, he was a research faculty
member in the Institute of Microelectronics at
Tsinghua University, Beijing, working on various
silicon-based chemical and mechanical sensors.
He is an Assistant Professor at the Department of Electrical and Computer
Engineering of the University of Florida, Gainesville. He spent summer 2001 at
the North America Research Center of Robert Bosch Corporation designing a
6-DOF inertial measurement unit. He has over 30 technical papers. His present
research interests include integrated microsensors, optical MEMS, optical
switching, biomedical imaging and sensing, and fiber-optic sensors.
Yingtian Pan is Assistant Professor of Biomedical
Engineering at SUNY Stony Brook. His current research is focused on biomedical imaging and bioinstrumentation. He has over 30 journal publications in
the related fields.
Gary K. Fedder (S’93–M’95–SM’01) received the
B.S. and M.S. degrees in electrical engineering from
the Massachusetts Institute of Technology (MIT),
Cambridge, in 1982 and 1984, respectively. In 1994,
he received the Ph.D. degree from University of
California at Berkeley, where his research resulted
in the first demonstration of multimode control
of a underdamped surface-micromachined inertial
device.
From 1984 to 1989, he worked at the
Hewlett-Packard Company on circuit design
and printed-circuit modeling. He is a Professor at Carnegie Mellon University
holding a joint appointment with the Department of Electrical and Computer
Engineering and The Robotics Institute. He has contributed to over 90 research
publications and several patents in the MEMS area. His research interests
include microsensor and microactuator design and modeling, integrated MEMS
manufactured in CMOS processes and structured design methodologies for
MEMS.
Dr. Fedder received the 1993 AIME Electronic Materials Society Ross Tucker
Award, the 1996 Carnegie Institute of Technology G.T. Ladd Award, and the
1996 NSF CAREER Award. Currently, he serves as a subject editor for the
IEEE/ASME JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, on the editorial board of the IoP Journal of Micromechanics and Microengineering and
as co-editor of the Wiley-VCH Sensors Update book series.